Re: [PATCH v2 1/2] abspath: add a function to resolve paths with missing components

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

[-- Attachment #1: Type: text/plain, Size: 4113 bytes --]

Hi brian,

On Sat, 10 Oct 2020, brian m. carlson wrote:

> On 2020-10-09 at 21:10:04, Junio C Hamano wrote:
> > "brian m. carlson" <sandals@crustytoothpaste.net> writes:
> >
> > > We'd like to canonicalize paths such that we can preserve any number of
> > > trailing components that may be missing.
> >
> > Sorry, but at least to me, the above gives no clue what kind of
> > operation is desired to be done on paths.  How would one preserve
> > what does not exist (i.e. are missing)?
> >
> > Do you mean some leading components in a path point at existing
> > directories and after some point a component names a directory
> > that does not exist, so everything after that does not yet exist
> > until you "mkdir -p" them?
> >
> > I guess my confusion comes primarily from the fuzziness of the verb
> > "canonicalize" in the sentence.  We want to handle a/b/../c/d and
> > there are various combinations of missng and existing directories,
> > e.g. a/b may not exist or a/b may but a/c may not, etc.  Is that
> > what is going on?  Makes me wonder if it makes sense to canonicalize
> > a/b/../c/d into a/c/d when a/b does not exist in the first place,
> > though.
>
> The behavior that I'm proposing is the realpath -m behavior.  If the
> path we're canonicalizing doesn't exist, we find the closest parent that
> does exist, canonicalize it (à la realpath(3)), and then append the
> components that don't exist to the canonicalized portion.

FWIW I was immediately able to think of a handful scenarios where this
functionality would come in handy, but I am probably not a typical example
for the median reader. So maybe a concrete example or two why this could
be handy could be shown in the cover letter?

Thanks,
Dscho

>
> > > Let's add a function to do
> > > that that calls strbuf_realpath to find the canonical path for the
> > > portion we do have and then append the missing part.  We adjust
> > > strip_last_component to return us the component it has stripped and use
> > > that to help us accumulate the missing part.
> >
> > OK, so if we have a/b/c/d and know a/b/c/d does not exist on the
> > filesystem, we start by splitting it to a/b/c and d, see if a/b/c
> > exists, and if not, do the same recursively to a/b/c to split it
> > into a/b and c, and prefix the latter to 'd' that we split earlier
> > (i.e. now we have a/b and c/d), until we have an existing directory
> > on the first half?
>
> Correct.
>
> > > +/*
> > > + * Like strbuf_realpath, but trailing components which do not exist are copied
> > > + * through.
> > > + */
> > > +char *strbuf_realpath_missing(struct strbuf *resolved, const char *path)
> > > +{
> > > +	struct strbuf remaining = STRBUF_INIT;
> > > +	struct strbuf trailing = STRBUF_INIT;
> > > +	struct strbuf component = STRBUF_INIT;
> > > +
> > > +	strbuf_addstr(&remaining, path);
> > > +
> > > +	while (remaining.len) {
> > > +		if (strbuf_realpath(resolved, remaining.buf, 0)) {
> > > +			strbuf_addbuf(resolved, &trailing);
> > > +
> > > +			strbuf_release(&component);
> > > +			strbuf_release(&remaining);
> > > +			strbuf_release(&trailing);
> > > +
> > > +			return resolved->buf;
> > > +		}
> > > +		strip_last_component(&remaining, &component);
> > > +		strbuf_insertstr(&trailing, 0, "/");
> > > +		strbuf_insertstr(&trailing, 1, component.buf);
> >
> > I may be utterly confused, but is this where
> >
> >     - we started with a/b/c/d, pushed 'd' into trailing and decided
> >       to redo with a/b/c
> >
> >     - now we split the a/b/c into a/b and c, and adjusting what is
> >       in trailing from 'd' to 'c/d'
> >
> > happens place?  It's a bit sad that we need to repeatedly use
> > insertstr to prepend in front, instead of appending.
>
> Yes, that's true.  It really isn't avoidable, though, with the functions
> the way that they are.  We can't use the original path and keep track of
> the offset because it may contain multiple path separators and we don't
> want to include those in the path.
> --
> brian m. carlson: Houston, Texas, US
>